How can I inherit from two classes with the same non-virtual grandparent classes?

Question

Suppose I have four classes A, B, C and D:

class A {
public:
    float someData;
    float someFunc();
}

class B : public A {
public:
    float someOtherData;
    float someOtherFunc();
}

class C : public A {
public:
    float evenMoreData;
    float evenMoreFuncs();
}

class D : public B, public C {
public:
    float andMoreData;
    float andMoreFuncs();
}

Now I want D to inherit only one instance of A. I know that this is diamond inheritance, which I could solve by adding a virtual to "public A" in B and C, but the problem here is that B and A belong to a library, which I can't change.

Is there any way to do this except from making B a member of D and wrapping all of B's member functions?

EDIT: As noted in the comments, in this case B would not need to inherit from A, but B also serves as a base class for other classes which by themself don't inherit any class that inherits A.

Answer 1

the problem here is that B and A belong to a library, which I can't change.

Ok, so that leaves the only option to change C and D.

You could make C into a class template which you can then use to either inherit from A or B - and in the case of D, you'll inherit from C<B> only:

template<class Base>
class C : public Base {
public:
    float evenMoreData;
    float evenMoreFuncs();
};

class D : public C<B> {
public:
    float andMoreData;
    float andMoreFuncs();
};

Demo

---

Another option is to drop the inheritance and to use composition. Here C<A> will have an A member variable and C<B> will have a B member variable:

template<class T>
class C {
public:
    T t;
    float evenMoreData;
    float evenMoreFuncs();
};

class D {
public:
    C<B> c;
    float andMoreData;
    float andMoreFuncs();
};

Demo